it could be used in audio as a clipping circuit (or clamping), for instance to "pre-clip" an audio signal before an amplifier, to protect the amplifier from input stage damage. some amplifiers (especially op amps) can be damaged if the input voltage exceeds the rail voltages. in many receiver designs, the inputs go to either op amps or CMOS selector switches, which might be damaged by voltage surges or even static electricity, and the zeners clamp the voltage to a safe level. there have also been amplifiers that use such a circuit at the input to clip the waveform slightly below the voltage required to saturate the output stage of the power amp. by keeping the output stage out of saturation, the amplifier and it's feedback loop don't "misbehave". the side effects of saturation can sound much worse than the clipped signal itself (as well as damaging tweeters). the technique is called "soft clipping"